03-05-2014, 10:39 AM
3G UMTS HSPA - High Speed Packet Access Tutorial
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3G HSPA, High Speed packet Access is the combination of two technologies, one of the downlink and the other for the uplink that can be built onto the existing 3G UMTS or W-CDMA technology to provide increased data transfer speeds.
The original 3G UMTS / W-CDMA standard provided a maximum download speed of 384 kbps.
With many users requiring much high data transfer speeds to compete with fixed line broadband services and also to support services that require higher data rates, the need for an increase in the speeds obtainable became necessary.
This resulted in the development of the technologies for 3G HSPA.
HSPA features
The system provides an enhancement on the basic 3G WCDMA / UMTS cellular system, providing data transfer rates that are considerably in excess of those originally envisaged for 3G as well as much greater levels of spectral efficiency.
Note on 3G UMTS / W-CDMA:
UMTS - Universal Mobile Telecommunications System is a 3G cellular system that uses Wideband CDMA, WCDMA as the format for the radio transmission. Its aim was to provide high speed data at much higher speeds than was previously possible. The basic system provided for speeds of 2 Mbps in the downlink and 384 kbps in the uplink.
3G HSPA benefits
The UMTS cellular system as defined under the 3GPP Release 99 standard was orientated more towards switched circuit operation and was not well suited to packet operation. Additionally greater speeds were required by users than could be provided with the original UMTS networks. Accordingly the changes required for HSPA were incorporated into many UMTS networks to enable them to operate more in the manner required for current applications.
HSPA provides a number of significant benefits that enable the new service to provide a far better performance for the user. While 3G UMTS HSPA offers higher data transfer rates, this is not the only benefit, as the system offers many other improvements as well:
1. Use of higher order modulation: 16QAM is used in the downlink instead of QPSK to enable data to be transmitted at a higher rate. This provides for maximum data rates of 14 Mbps in the downlink. QPSK is still used in the uplink where data rates of up to 5.8 Mbps are achieved. The data rates quoted are for raw data rates and do not include reductions in actual payload data resulting from the protocol overheads.
2. Shorter Transmission Time Interval (TTI): The use of a shorter TTI reduces the round trip time and enables improvements in adapting to fast channel variations and provides for reductions in latency.
3. Use of shared channel transmission: Sharing the resources enables greater levels of efficiency to be achieved and integrates with IP and packet data concepts.
4. Use of link adaptation: By adapting the link it is possible to maximize the channel usage.
5. Fast Node B scheduling: The use of fast scheduling with adaptive coding and modulation (only downlink) enables the system to respond to the varying radio channel and interference conditions and to accommodate data traffic which tends to be "bursty" in nature.
6. Node B based Hybrid ARQ: This enables 3G HSPA to provide reduced retransmission round trip times and it adds robustness to the system by allowing soft combining of retransmissions.
Beyond 3G HSPA
With the elements of 3G HSPA launched, further evolutions were in the pipeline. The first of these was known as HSPA+ or Evolved HSPA. The evolved HSPA or HSPA+ provides data rates up to 42 Mbps in the downlink and 11 Mbps in the uplink (per 5MHz carrier) which it achieves by using high order modulation and MIMO (multiple input, multiple output) technologies.
UMTS HSPA and 3GPP standards
The new high speed technology is part of the 3G UMTS evolution. It provides additional facilities that are added on to t e basic 3GPP UMTS standard. The upgrades and additional facilities were introduced at successive releases of the 3GPP standard.
Release 4: This release of the 3GPP standard provided for the efficient use of IP, a facility that was required because the original Release 99 focussed on circuit switched technology. Accordingly this was a key enabler for 3G HSDPA.
Release 5: This release included the core of HSDPA itself. It provided for downlink packet support, reduced delays, a raw data rate (i.e. including payload, protocols, error correction,
HSDPA Hybrid ARQ and soft combining
Hybrid ARQ or HARQ is hybrid automatic repeat request and it is essentially a form of the more common ARQ error correction methodology. When the basic ARQ format is used, error-detection information bits are added to data to be transmitted. One form of this may be a cyclic redundancy check, CRC. However when Hybrid ARQ is used, forward error correction (FEC) bits are also added to the existing error detection bits. The added error detection means that Hybrid ARQ performs better than ordinary ARQ in poor signal conditions, but the additional overhead can reduce the throughput in good signal conditions.
The combination of Fast Hybrid ARQ and soft combining enables the terminal to request the retransmission of data that may be received erroneously. This can be done within the adaptive modulation and channel coding scheme so that when error-rates rise the link can be modified accordingly.
The user equipment or terminal receives the data and decodes it, reporting back the result to the NodeB after the reception of each block, and in this way rapid retransmission of any blocks with errors can be undertaken. This significantly reduces delays, especially under poor radio link conditions or when the link is changing rapidly.
Soft combining is a process whereby the user equipment or terminal does not discard information it cannot decode. Instead it retains it to combine with any retransmission data to increase the chance of successful decoding of the data.
A process called Incremental Redundancy (IR) is also used with the retransmissions. This process adds additional parity bits in retransmissions to make the data retransmission more robust.
HSDPA performance
Using HSDPA scheme it will be possible to achieve peak user data rates of 10 Mbps within the 5 MHz channel bandwidth offered under 3G UMTS. The new scheme has a number of benefits. It improves the overall network packet data capacity, improves the spectral efficiency and will enable networks to achieve a lower delivery cost per bit. Users will see higher data speeds as well as shorter service response times and better availability of services. However new mobile designs will need to be able to handle the increased data throughput rates. Reports indicate that handsets will need to have at least double the memory currently contained within handsets. Nevertheless the advantages of 3G HSDPA mean that it will be widely used as networks are upgraded and new phones introduced